A MIMO (multi-input multi-output) technology is a technology configured to enhance efficiency of data transmission and reception using a plurality of transmitting antennas and a plurality of receiving antennas instead of using a single transmitting antenna and a single receiving antenna. If a single antenna is used, a receiving side receives data via a single antenna path. On the contrary, if multiple antennas are used, the receiving end receives data via various paths. Hence, by using the MIMO, data transmission speed and an amount of data transmission can be enhanced and coverage can also be enlarged.
A single-cell MIMO operation can be classified into a single user-MIMO (SU-MIMO) scheme and a multi user-MIMO (MU-MIMO) scheme. The SU-MIMO is a scheme that a single user equipment receives a downlink signal in one cell. The MU-MIMO is a scheme that two or more user equipments receive a downlink signal in one cell.
Meanwhile, a study on a CoMP (coordinated multi-point) system to improve throughput of a user positioned at a cell boundary in a manner of applying an improved MIMO transmission in a multi-cell environment is actively progressing. If the CoMP system is applied, inter-cell interference can be reduced in the multi-cell environment and it may enhance overall performance of a system.
Channel estimation indicates a process of restoring a received signal in a manner of compensating for distortion of a signal generated by fading. In this case, the fading indicates a phenomenon that strength of a signal rapidly changes due to multi path-time delay in a wireless communication system environment. In order to perform the channel estimation, it is necessary to have a reference signal known to both a transmitter and a receiver. And, the reference signal can be simply called an RS (reference signal) or a pilot depending on an applied standard.
A downlink reference signal is a pilot signal used for a coherent demodulation and includes PDSCH (physical downlink shared channel), PCFICH (physical control format indicator channel), PHICH (physical hybrid indicator channel), PDCCH (physical downlink control channel), and the like. The downlink reference signal can be classified into a common reference signal (CRS) shared by all user equipments in a cell and a dedicated reference signal (DRS) used for a specific user equipment only. Compared to a legacy communication system (e.g., a system following LTE release 8 or 9 standard) supporting 4 transmitting antennas, a system (e.g., a system supporting 8 transmitting antennas and following LTE-A standard) including an extended antenna configuration considers DRS-based data demodulation to manage an efficient reference signal and support a developed transmission scheme. In particular, in order to support data transmission via an extended antenna configuration, it is able to define a DRS for two or more layers. Since the DRS and data are precoded by an identical precoder, it may be easy for a receiving side to estimate channel information necessary for demodulating data without separate precoding information.
Meanwhile, the downlink receiving side is able to obtain precoded channel information for the extended antenna configuration via the DRS. Yet, in order to obtain channel information which is not precoded, it is necessary to have a separate reference signal except the DRS. Hence, the receiving side can define a reference signal to obtain channel state information (CSI), i.e., CSI-RS in the system following the LTE-A standard.